In the cellulose and paper industries, for example, it is necessary to be able to form webs of fiber suspensions in an even and wide distributed flow in the transverse and longitudinal directions on a base, such as a roll, drum or the like. An uneven formation may thus result in an impaired pulp quality, for example due to fiber damages at subsequent press nips in thicker formed sections, canalization of the washing liquid, and poor efficiency during displacement washing.
Distribution of the flow of the flowing medium is controlled substantially by frictional losses (i.e. pressure drop) when the medium flows through a distributor. In order to ensure an even distribution, propagation and discharge of the medium in the transverse direction along a long and narrow gap, e.g. in a rectangular shaped distribution section, which is often desired, any of the following two principles mentioned can be applied:
Design the distributor such that the pressure drop along each streamline, for an evenly distributed outlet flow, from the inlet to the outlet, become essentially the same.
Provide a large pressure drop at the outlet of the distributor such that the differences in friction losses along different streamlines become negligible compared to the outlet friction losses.
One problem in applying the first principle (1) above is that the variation in velocity along individual streamlines of the flowing medium is hard to predict. This fact in combination with limited knowledge about the boundary layer behavior of e.g. suspensions of wood fibers, makes it difficult to predict the pressure drop along the streamlines. One problem is clogging of the distributor when the fibers tend to slow down or adhere to the inner faces of the distributor which influences the runnability. Known distributors have also been shown to be sensitive to variations in the flow velocity.
One object of the present invention is to provide a method and an apparatus according to the first principle, where an improved propagation and distribution of a flowing medium is accomplished and where the above mentioned problems are minimized.